With the motivation to switch to more efficient methods of generating light, such as use of fluorescent lamps, a need exists to provide features such as dimming at an economical cost. A typical resonant circuit fluorescent lighting ballast and fluorescent lamp are shown in FIG. 1. Operation may be understood by representing this circuit as two equivalent resistor-inductor-capacitor (RLC) circuits. The first equivalent circuit, shown in FIG. 2, is series resonant at a particular frequency, selection of which depends on the choice of components and control resolution of an oscillator circuit. For example, a frequency may be selected at about 70 kHz which will be the series resonance of the inductor 110 and the filament capacitor 116 (Cf). The second equivalent circuit is shown in FIG. 3. Note that in both equivalent circuits the capacitor 114 (C) has been replaced by a short circuit (zero resistance). The function of the capacitor 114 is to perform DC blocking (allowing only AC signals through the circuit) and is chosen to have a high value of capacitance for this purpose. It is modeled to be a short (low impedance connection at the AC signal frequencies) in these equivalent circuits.
When the fluorescent lamp 112 is off, the ballast is first driven at frequency, FHigh. This frequency is chosen to be above the resonant frequency point of the RLC circuit, and is design specific, but may be for example purposes about 100 kHz. At this frequency, FIG. 2 best represents the lamp's equivalent circuit since the lamp gas has not yet ionized. The frequency response of the circuit with respect to the current is shown in FIG. 4. The purpose here is to run current through the filaments of the lamp, this is typically referred to as the ‘Preheat’ interval (1). When the filaments are warm enough to ionize the surrounding lamp gas, the drive frequency is lowered. This causes the RLC circuit to be swept near its resonant frequency, causing an increase in the voltage across the lamp. An arc will occur in the lamp at its ‘strike’ voltage (2) and the arc will ignite (ionize) the gas.
Lamp ‘ignition’ means that the gas is now ionized enough to conduct an electric current. The lamp 112 is now said to be on (producing visible light). At this point, FIG. 3 best describes the behavior of the lamp ballast circuit. Note that the lamp 112 now behaves as an L in series with a parallel R and Cf. The R in this case is the electrical resistance of the ionized gas in the lamp 112 and Cf is the filament capacitance 716. Once the lamp 112 is ignited the voltage stays fairly constant, but the light intensity from the fluorescent lamp(s) will vary as frequency thereto changes. A typical useful dimming range may occur from about 50 KHz to about 100 KHz, shown as the second plot curve (3) of FIG. 4. As more current flows through the fluorescent lamp (higher voltage across the filaments of the lamp 112, the greater the light intensity. The current flowing through the lamp 112 can be controlled by adjusting the frequency of an input signal to the lamp 112. The lamp 112 and reactive circuit may be driven by a pair of power transistors 106 and 108 that are typically external to a control device 120. All of the other elements within the box are typically part of the control device 120. The power transistors 106 and 108 are driven in a complementary fashion so that the top transistor 106 is on for part of a period, T, and the bottom transistor 108 is on for the remainder of the period. A dead time interval is used between the on times so that both power transistors 106 and 108 are never conducting at the same time (see FIG. 8).
To control the fluorescent lamp, the dead time unit must receive a variable frequency signal with a duty cycle of about 50%. A signal may be provided in a microcontroller based application by a pulse width modulation (PWM) generator in combination with a clock, e.g., resistor capacitor (RC) oscillator. The PWM generator has the ability to generate digital signals with controllable variable frequency and duty cycle. The frequency of the PWM signal is adjusted by changing the value of a PWM period register, while the duty cycle is maintained at substantially fifty (50) percent by changing the value of a PWM duty register (see FIG. 6).
Florescent light ballast manufacturers require ultra high frequency resolution to provide smooth and accurate dimming control of the fluorescent lamps. The frequency step resolution of the PWM generator is a function of the input clock frequency thereto and the desired lamp excitation frequency. However, in typical PWM generator applications, the PWM period register adjustment is not capable of producing small enough frequency steps for precise control of the lamp current (light intensity). In order to provide such resolution, for example at 100 kHz, it would require a pulse width modulation (PWM) generator, used for controlling the fluorescent lamp dimming, to be driven with a clock frequency in excess of 50 MHz.